The invention relates to line blades for a rotating trimmer and, more particularly, to a multi-zone line blade with improved aerodynamic, stability and cutting capabilities.
Monofilament line, commonly used with all battery trimmers and most gas trimmers, is inherently inefficient and weak due to (1) excessive power drag inefficiencies, (2) limited and poor cutting quality and performance, which requires higher speed cutting, (3) excessive wear and failure rates needing constant replenishment from unreliable bump systems, and (4) limited ability to cut bigger vegetation. Monofilament line is by nature over-stressed, and its complex and unreliable feeding systems are designed to compensate for these well-known and continuing monofilament line failures. Users are highly frustrated with monofilament line and feed (bump) systems because of their inherent respooling and jamming complexities. Further disappointment ensues with the performance of cordless (battery-powered) trimmers, as grass cutting quantity and quality with monofilament line are inferior, and most often, do not meet users' job needs in cutting time due to limited battery life. Battery powered attempts do not have an adequate charge and do not run long enough to finish the workload. Further, battery life is limited, they are heavy creating user fatigue, and they are costly to replace.
The current state of the art for flexible line blades provides a major reliability and cutting improvements over monofilament line, particularly regarding its better ability to cut at much lower speeds while being more adequately designed to withstand normally high impact forces without the harassing failures of monofilament lines. Further and more importantly, the line blade orientation can be pitched to create lift-induced updraft that assists with grass lift in the rotary mowing process. While a major improvement in so many areas, there are numerous tradeoff problems with existing line blades associated with high drag, flapping, twist resonance, and other grass cutting characteristics.
Current line blade designs incorporate three basic segment/zones of different shapes and configurations, each serving individual operational performance and function. The blade segment may carry a sharp leading edge from the hinge to the cutting tip. This inherently produces a vulnerability to aerodynamic and aeroelastic forces that dramatically change how the blade interacts with related active forces, thus making it susceptible to instability. The three-segment line blades, such as the line blades available from Aero-Flex Technologies in Rock Hill, S.C., are growing in acceptance and are beneficial for gas trimmers. The existing line blades, however, are not yet fully practical for cordless trimmers because adequate battery charge storage technology is currently limited, making such trimmers ‘power starved,’ and in order for the current line blades to cut with a leading sharper edge, which has been found desirable through testing and experimentation, an elevated drag further reduces battery life. While cutting on the blunt side of the aerodynamically-shaped cross-section flexible line blades is still superior to using a monofilament line, the added benefits of cutting with the sharper side of the cross-sectional shape are significant.
Cordless trimmers are far from being competitive with gas trimmers. High energy concentration of gasoline in relatively large fuel tanks enables gas trimmers to allow wasted energy. As long as there is ample fuel available, gas trimmers are not subject to energy limitations, and because refilling the fuel tank is a short process. On the other hand, cordless trimmers' use time is capped due to the very limited supply of energy stored in batteries, which can take hours to recharge, effectively rendering the tool useless until recharged. While battery technology is improving, it falls far short of gasoline energy. For example, it takes at least ten (10) 120 watt-hour batteries to equate the energy in one pint of gasoline, generally about a tank in a gas trimmer. Therefore, so that user run time can be maximized, inefficient use of energy in cordless trimmers needs to be minimized and this includes the high drag penalties of cutting with conventional monofilament line or inefficient and unstable line blades. While cordless trimmers are an attraction over gas trimmers (no gas, no smoke, low noise, no cranking, etc.), current user dissatisfaction from poor performance from monofilament line further implies an inferior or lower value product to the user.
While the existing flexible line blades from Aero-Flex Technologies will improve trimming well beyond monofilament line, there still remains negatives to be resolved by the line blades of the described embodiments. There is a need to enable more cordless trimmer options toward better cutting rates, greater area cut per battery charge, and higher feed speed options for more aggressive trimming line blade flutter, a current disadvantage, is due to complex interaction of aerodynamic and torsional forces acting at different order harmonics. Flutter is a flapping of the blade better described as similar to a bird's wing flapping. It severely interferes with cutting vegetation, especially when assisted with a glider support disc during mowing with flexible line blades. Flutter also creates aggressive impact of the blades to the grass to prevent an even height or level cut. Flutter also creates unequal wear between the line blades, creating more vibratory effects on the trimmer head and on the user.
Added noise from sharp leading edges with existing line blades is generated from harmonic resonance conditions due to complex dynamic and interactive forces such as generated by turbulence, air bending forces, line blade structure, material torsional resistance, and centrifugal forces. These unstable harmonic twists are due to the complex interaction of aerodynamic and centrifugal forces further complicated by the line blade torsional response (like a torsional spring). This has been observed with a strobe light together with dynamometer testing, as well as a study of high speed camera viewing. Close examination shows that (but occurring less when cutting into vegetation) pitch angle cycles dynamically between positive and negative twisting due to complex differentiating operational forces on the blade further influenced by torsional cycling. These reactions increase drag and energy losses. When free spinning (but less occurring when into cutting into vegetation), these added dynamic deficiencies create an undesirable and irritating noise at elevated pitch levels. Heavy amperage overloading also occurs to cause current ‘spiking,’ a premature circuit breaker overload activation shutting down motor operation. All these negatives severely hinder the current state of the art of line blade usage for cordless trimmers.